"Plasma technologies"

RESEARCH AND PRODUCTION COMPANY

users.iptelecom.net.ua/~ytyurin/english/Introduction-1.htm

Surface modification can be performed by different methods involving treatment with concentrated energy flows. These are laser, electron and ion beams, shock waves, as well as stationary and pulsed plasma flows. If the surface of the material treated is covered by thin films of a different material, the short-time effect by energy flows leads to mixing and re-distribution of elements contained in a film and substrate, in addition to phenomena of a hardening character. As a result, stable and meta-stable alloys and compounds are formed. Super-high cooling and heating rates combined with the effect of physical fields allow nano-, microcrystalline or amorphous layers to be produced on the surface. Properties of these layers may be substantially different from those of metals and alloys in the equilibrium state.

Modification of properties of the surface is based on the principle of formation of complex multi-layer surface structures, such as amorphous films, finely dispersed precipitates and sub-layers with a high dislocation density. This allows a targeted variation of service characteristics of metallic materials, including corrosion resistance, strength, wear resistance, fatigue strength under cyclic loads, electric-erosion resistance, etc.

It should be noted that, along with experience of employing laser and electron beam treatment, a large amount of experimental data has been accumulated on the use of the high-energy density methods for treatment of materials with ion and plasma jets.

Main premises for using high-energy density plasma technologies are as follows:

-high energy density within the treatment zone and possibility of regulating it;

-short time and locality of the effect limiting the heat-affected zone;

-simplicity of control of energy pulses and possibility of treating curvilinear surfaces by no-contact methods;

-low costs of equipment and technology;

-possibility of automating the treatment process;

-high factor of utilisation of power consumed for hardening.

The surface of a workpiece is treated during the process of plasma modification by thermochemical and thermomechanical effects that stimulate plasma-chemical synthesis and formation of new compounds. This allows deposition of coatings with predictable properties.

In addition, this allows alloying of the workpiece surface and formation of nano-crystalline structures having high wear, heat and scoring resistance, combined with sufficient strength and toughness of an alloy used to make a part.

The page contains information on optimisation of the surface geometry by vibration work-hardening, which provides the possibility of lubrication of rubbing surfaces. In this case the hydrodynamic effect of lubrication works, thus enhancing the effect of surface modification and complementing the effect of treatment with highly concentrated plasma flows. Optimisation of the surface geometry leads to multiple decrease of friction and wear coefficients in sliding friction pairs.

Treatment of the workpiece surface with an optimal geometry by using intensive energy beams offers an extra possibility of controlling properties of the surface.

Optical quantum generators, i.e. lasers, and different types of plasma accelerators have been developed in the last decades and are commercially applied now for this purpose. Combined with the shock wave effect, heating and deformation, these devices can provide the radiation and electromagnetic effects on surfaces. Plasma accelerators have a number of advantages over lasers, such as a much higher efficiency, lower power consumption per unit surface being modified and high degree of absorption of radiation in any material.

Methods involving the shock effect on the surface by using explosives are also efficient. Explosives form plasma jets with a velocity of 5-20 km/s, temperature of 20,000-60,000 K and shock wave pressure of 300-2,000 MN/m². The density of plasma with these methods is higher than that of the atmosphere by a factor of 50. Plasma of the explosives is also employed for thermochemical treatment of surfaces. Work is progress on further development of methods for generating pulsed plasma by electric explosion of conductors. Plasma of the electric discharge is a versatile tool. It is used for thermochemical treatment of workpiece surfaces under the layer of liquid and in air atmosphere. Dozens of pulsed plasma duplex technologies are available. These technologies combine the high-rate heat treatment and alloying of surfaces with plasma components, such as carbon or nitrogen. Analysis of the investigation results shows that the impact by pulsed plasma on solid surfaces is more efficient than treatment with stationary jets, which is the case, for example, of "laser plasma".

The page contains data on development of the technologies based on the use of non-stationary electric discharges in plasma jets. Formation of a plasma jet takes place under the effect of non-stationary detonation waves propagating between symmetric electrode units. In this case the energy parameters of the plasma can be controlled by a fuel mixture composition, electric potential and geometric characteristics of a device. Parameters of the pulsed plasma can be varied within the following ranges: energy – 1,000-10,000 J, frequency – 1-10 Hz, duration – 0.5x10^-3-5x10^-3 s, velocity – 2,000-9,000 m/s and temperature – 5,000-30,000 K. Electric current of up to 15 kA is fed to the workpiece surface via the plasma jet, thus forming the magnetic and acoustic fields.

We suggest affecting the workpiece surface by a high-energy flow of alloying elements. This results in rapid heating (heating time – 10^-3-10^-6 s) of the surface layer, followed by its intensive cooling through removing heat both into the bulk of metal and to the surrounding atmosphere. The heat effect here is combined with the alloying processes.

The high rate of heating and cooling (10⁴-10⁸ K/s) of the surface layer of metal leads to formation of nano-microcrystalline structure, high dislocation density and growth of the concentration of alloying elements.

The pulsed-plasma technology allows a simultaneous, in one treatment pulse, realisation of different methods of affecting the workpiece surface: elasto-plastic deformation, impact by sound and pulsed magnetic field, heat and electric-pulse treatment, and deformation of metals and alloys during reversible transformations.

High power density of the flow (up to 10⁷ W/cm² at the point of contact with the workpiece surface) makes it possible to perform treatment in air atmosphere with no surface preparation.

Treatment with a high-energy density flow of alloying elements causes no changes in geometric sizes of workpieces. Therefore, it is suggested that it should be used as a finishing operation. Industrial verification shows that performance of tools and machine parts after modification of their working surfaces increases 3-5 times.

Depending upon the composition of the high-energy density flow, the surface layer can have high anti-friction properties, as well as high heat, wear and corrosion resistance. As proved experimentally, the friction coefficient after treatment with the high-energy density flow decreases 3-5 times, load to seizure increases 10 times, wear resistance under fretting grows 2-5 times, and heat resistance increases 6 times.

The technology allows using low alloys with high strength and ductility for the fabrication of parts. Along with reduction in cost of a workpiece material, this leads to reduction in machining and heat treatment costs.

The page also contains data on modification of surfaces by using electric discharges in a layer of electrolytic plasma. Analysis of processes occurring in the plasma layer between the liquid anode and a solid surface of the cathode is presented. The technology for electrolytic-plasma hardening (EPH) of surfaces is described, wherein the process of formation and maintaining of the plasma layer is stabilised by a periodic increase of electric voltage. The heating rate can be adjusted within a range of 20 to 500 ^oC/s through varying the time of connection of an increased electric potential, thus enabling formation of the hardened layers with thickness ranging from 0.1 to 10 mm. Adding alloying elements to electrolyte makes it possible to perform thermochemical treatment of the surface heated.

With the EPH process, the plasma material is a water-based electrolyte, and cooling of the heated surface is provided using the same electrolyte. Configuration of the hardened layer depends upon the design of a heater and may have the shape of a circle, square, ring or ellipse. The density of power used for heating the surface may range from 1x10³ to 1x10⁴ W/cm², which is enough for the surface heat treatment, of the type of laser treatment.

The electrolytic-plasma hardening method offers the following benefits:

1)    it provides a hardened layer from 0.1 to 10 mm deep on the workpiece surface;

2)    it allows formation of highly dispersed structures on the workpiece surface, providing a layer with hardness of 10-19 GPa;

3)    it can be used for local hardening of regions on the workpiece surface, which are subjected to the most intensive wear, thus avoiding heating of an entire part;

4)    it provides high economic indices owing to a low cost, simplicity and availability of the involved equipment, as well as high productivity of the process;

5)    the technology used is environmentally clean, and the treatment process can be readily automated.

The range of parts for plasma hardening can be conditionally subdivided into the following three groups. These are parts for which the known (baseline) hardening methods fail to increase strength to a required level, parts that should have a combination of high operating properties (hardness, wear resistance, resistance to fracture through cracking), and parts the strength of which cannot be increased by the known and commercially verified technologies, or for the treatment of which the available technologies and equipment are two expensive.

The page describes technologies and equipment that are commercially applied for modification of working surfaces of machine parts and tools. Examples of application of the technologies in metallurgy, mining industry, wood working, machine building and other industrial sectors are given.